Hydroxylamine sulfate (HAS) is an important chemical intermediate, mainly used in the synthesis of anticancer drugs (hydroxyurea), sulfonamides (sulfamethoxazole), and pesticides (methomyl), and has a large market demand and wide application value. Currently, traditional production processes for Hydroxylammonium sulfate mainly include the nitromethane route, natural gas nitration, ammonium disulfonate hydrolysis, nitric oxide catalytic reduction, and ketoxime hydrolysis. The advantages and disadvantages of each process are as follows:
To further reduce energy consumption and wastewater discharge in hydroxylamine sulfate production, and addressing the issue of high water content in the reaction solution during oxime hydrolysis to prepare hydroxylamine salts, this paper calculates the theoretical residual water volume and the mass of hydroxylamine sulfate produced at a certain conversion rate under different sulfuric acid concentrations. Combined with the results of solubility measurements of hydroxylamine sulfate in water at atmospheric pressure, this paper proposes coupling two reaction separation technologies—reaction-distillation and reaction-crystallization—for the ketoxime hydrolysis to hydroxylamine sulfate production process. This improves the reaction conversion rate by continuously removing the product hydroxylamine sulfate and the byproduct ketone, while avoiding excessively high viscosity of the reaction solution that could affect mass and heat transfer, preventing localized overheating, and avoiding side reactions. This enables the preparation of hydroxylamine sulfate under a high-concentration sulfuric acid reaction system.
1. Experimental Section
1.1 Main Instruments and Reagents
Butanone oxime, purity 99.5%; Sulfuric Acid, purity 98%, prepared as a sulfuric acid aqueous solution of a certain concentration with deionized water; Ethanol, purity > 99.7%; Sodium Hydroxide, purity > 96%, prepared as a 0.2 mol·L⁻¹ sodium hydroxide solution. A UV spectrophotometer was used to measure the oxime content in the reaction solution and distillate; a smart vacuum pump was used for the reduced-pressure reactive distillation process.
1.2 Experimental Procedure
Figure 1 shows a schematic diagram of the experimental process flow, where the distillation column is a packed distillation column designed and assembled by the research group. The experimental steps are as follows:
(1) Prepare ketoxime and sulfuric acid solution for later use. Prepare a 40% (wt) ~ 60% (wt) sulfuric acid aqueous solution.
(2) Add ketoxime and sulfuric acid aqueous solution to the reaction vessel at a certain molar ratio of ketoxime and H2SO4. Control the temperature of the reaction solution at about 80℃. Adjust the rising steam volume by adjusting the vacuum degree. Collect the distilled ketone/water mixture from the top of the distillation column. When hydroxylamine sulfate crystals precipitate in the reaction solution, filter the reaction solution quickly. Alternatively, the reaction solution can be rapidly cooled and filtered. The filtered reaction solution is recycled and the reaction continues.
(3) After the reaction is completed, the vessel liquid is cooled, crystallized, filtered, washed, and dried to obtain solid hydroxylamine sulfate. The filtered mother liquor can be recycled for the next batch of reaction. The yield of hydroxylamine sulfate is obtained by analyzing the reaction solution and crystals.
2 Results and Discussion
2.1 Solubility of Hydroxylamine Sulfate in Water
Figure 2 shows the solubility curves of hydroxylamine sulfate in water at different temperatures. This paper uses the equilibrium method to measure the solubility of hydroxylamine sulfate. After stirring for 1 h to allow hydroxylamine sulfate to fully dissolve and reach equilibrium, the mixture was allowed to stand for 30 min after stirring stopped. The supernatant was then titrated to determine the solubility of hydroxylamine sulfate in water at different temperatures. Based on the curves in the figure, the solubility of hydroxylamine sulfate in water at room temperature (20℃) is 55.87 g, classifying it as readily soluble. Furthermore, temperature changes significantly affect the solubility of hydroxylamine sulfate. Therefore, in the oxime hydrolysis preparation process, the yield of hydroxylamine sulfate crystals can be increased by lowering the crystallization temperature of the mother liquor. Simultaneously, lowering the temperature also reduces the viscosity of the reaction solution, preventing excessive viscosity from affecting mass and heat transfer.
2.2 Selection of Reaction Process Enhancement Methods
Figure 3 shows the curves for residual water, generated hydroxylamine sulfate, and soluble hydroxylamine sulfate at a theoretical conversion rate of 98% for oxime hydrolysis under different sulfuric acid concentrations. The residual water volume variable w is calculated based on the azeotropic composition of butanone and water (0.96). As shown in Figure 3, after the initial sulfuric acid concentration reaches 37% (wt), the amount of hydroxylamine sulfate soluble in the remaining water content is less than the mass of hydroxylamine sulfate produced. That is, under the assumed conditions (oxime ratio 0.5:1, theoretical conversion rate 98%, and butanone mass fraction in the distillate 96%), hydroxylamine sulfate crystals precipitate after the initial sulfuric acid concentration exceeds 37% (wt). Simultaneously, as the initial sulfuric acid concentration increases, the remaining water content decreases significantly, which is beneficial for reducing distillation energy consumption and wastewater volume in the oxime hydrolysis preparation of hydroxylamine sulfate. Therefore, this paper couples two reaction separation technologies—reaction-distillation and reaction-crystallization—for the hydrolysis of ketoxime to prepare hydroxylamine sulfate.
2.3 Selection of Ketooxime Types
Table 2 shows the physical properties of commonly used ketoxime/ketone and water ratios under normal pressure (101.325 kPa). Since the hydrolysis of oxime to prepare hydroxylamine sulfate is a reversible reaction with a low equilibrium conversion rate, it is necessary to continuously remove the byproduct ketone to improve the yield of hydroxylamine sulfate. Table 2 shows that cyclohexanone has a higher boiling point than water, making water easier to remove. Since water is one of the reactants, removing large amounts of water is detrimental to improving the yield of hydroxylamine sulfate. Furthermore, removing large amounts of water leads to excessively high viscosity of the reaction solution, affecting mass and heat transfer efficiency. Butanone oxime and acetone oxime have higher boiling points than water, while Methyl Ethyl Ketone (MEK) and Acetone have lower boiling points, avoiding these adverse factors. However, the equilibrium conversion rate of the hydrolysis reaction of acetone oxime is extremely low, and the degree of oximation of the reverse reaction between acetone and hydroxylamine sulfate can approach 100%. Therefore, the preparation of hydroxylamine sulfate using the acidic hydrolysis of acetone oxime (i.e., the reverse reaction of oximation) is difficult, often accompanied by high energy consumption and low yield. Thus, the feasibility of preparing hydroxylamine sulfate via the acetone oxime hydrolysis route is poor. Therefore, this paper uses a butanone oxime/butanone system for process improvement verification.
2.4 Effect of Acid-Oxime Ratio
Figure 4 shows the effect of different acid-oxime ratios on the yield of hydroxylamine sulfate. As shown in the figure, the yield of hydroxylamine sulfate continuously increases before the acid-oxime ratio reaches 0.5:1. This is because the theoretical upper limit of oxime conversion increases with the increase of the acid-oxime ratio. However, when the acid-oxime ratio reaches 0.5:1, the theoretical yield of hydroxylamine sulfate reaches 100%, and the overall yield remains essentially unchanged. But when the acid-oxime ratio is less than 0.5:1, the loss of oxime in a single reaction is relatively high, mainly due to a large excess of butanone oxime in the later stages of the reaction, leading to an increased concentration of butanone oxime in the reaction solution. However, excess sulfuric acid reduces its atom utilization rate, lowering the atom economy of the process. In summary, an acid-oxime ratio of 0.5:1 is most suitable for the process of preparing hydroxylamine sulfate by oxime hydrolysis.
2.5 Effect of Sulfuric Acid Concentration
Figure 5 shows the effect of different acid concentrations on the yield of hydroxylamine sulfate. As can be seen from the figure, the yield of hydroxylamine sulfate did not decrease significantly with the increase of sulfuric acid concentration. This is because when a large amount of hydroxylamine sulfate crystals precipitate, the crystals are filtered in time to avoid the reaction liquid from affecting the mass and heat transfer efficiency due to excessive viscosity, thus preventing local overheating and avoiding side reactions. At the same time, the loss rate of methyl ethyl ketone oxime did not change significantly with the increase of sulfuric acid concentration. This is because the high conversion rate of methyl ethyl ketone oxime prevents the oxime concentration in the reaction liquid from becoming too high, reduces the concentration difference of methyl ethyl ketone oxime between the top and bottom of the column, and effectively inhibits the distillation of methyl ethyl ketone oxime. In the patent published by Lin Yong, the yield of hydroxylamine sulfate was 90% when the initial sulfuric acid concentration was 40% (wt) and the acid-oxime ratio was 1.2:1 after enhanced reaction-distillation coupling of the oxime hydrolysis reaction. In this experiment, the yield of hydroxylamine sulfate reached 95.59% after enhanced reaction-distillation and reaction-crystallization reaction separation technologies at an initial sulfuric acid concentration of 40% (wt) and an acid-oxime ratio of 0.5:1. This improved the atom utilization rate of sulfuric acid and greatly enhanced the atom economy of the process. Furthermore, the improved oxime hydrolysis process for preparing hydroxylamine sulfate, while ensuring no significant reduction in the yield, achieved the preparation of hydroxylamine sulfate in a high-concentration sulfuric acid reaction system (initial sulfuric acid concentration of 40%~60% (wt)), significantly reducing the water content of the reaction system. This not only reduces the energy consumption for hydroxylamine sulfate preparation but also reduces wastewater discharge, improving the environmental protection level and industrial application prospects of the process.
2.6 Improvement of the Coupling Process of Reaction-Distillation and Reaction-Crystallization
As shown in 2.5, the viscosity and solid content of the reaction solution have most adverse effects on the liquid-phase mass and heat transfer of the methyl ethyl ketone oxime hydrolysis to hydroxylamine sulfate system. However, the scouring of the thermal boundary layer by solid particles reduces the boundary layer thermal resistance, thus increasing the heat transfer coefficient. Based on these conclusions, this paper proposes the following improvement to the enhanced process of methyl ethyl ketone oxime hydrolysis to hydroxylamine sulfate using the coupling of reaction-distillation and reaction-crystallization:
The improved process of enhanced process of methyl ethyl ketone oxime hydrolysis to hydroxylamine sulfate using the coupling of reaction-distillation and reaction-crystallization employs two or more reaction-distillation units connected in series. A crystallization vessel is added between the two reaction-distillation units to cool and crystallize the reaction solution. When the solid content of the methyl ethyl ketone (MEK) oxime hydrolysis system is low, the reaction solution can be transferred to a crystallizer for cooling and crystallization. On the one hand, cooling and crystallization reduces the viscosity of the reaction solution, improving liquid-phase mass and heat transfer rates and enhancing process safety. On the other hand, it increases hydroxylamine precipitation, thus increasing the overall yield of the MEK oxime hydrolysis process. The improved reaction-distillation and reaction-crystallization coupling can be operated both batch and continuously.
After the process improvement, the initial concentration of added sulfuric acid was increased from 60% (wt) to 65% (wt), reducing the water content in the reaction system and also lowering the energy consumption of the distillation process. For every ton of hydroxylamine sulfate produced, the improved process requires 6.6 × 10⁴ kJ of energy for distillation, while the original process requires 1.8 × 10⁵ kJ. Compared to the original process, the improved process significantly reduces the energy consumption of the distillation process.
3. Conclusion
The coupling of reaction-distillation and reaction-crystallization techniques for the hydrolysis of ketoxime to prepare hydroxylamine sulfate improves the reaction conversion rate by continuously removing the product hydroxylamine sulfate and the byproduct ketone. This allows for the preparation of hydroxylamine sulfate in a high-concentration sulfuric acid reaction system (initial sulfuric acid concentration 40%~60% (wt)). Simultaneously, it avoids the impact of excessively high viscosity on mass and heat transfer, prevents local overheating, avoids side reactions, and improves product stability. The high initial sulfuric acid concentration avoids excessive water accumulation leading to increased distillation and subsequent byproduct purification energy consumption, effectively reducing energy consumption and wastewater discharge in the oxime hydrolysis to hydroxylamine sulfate preparation process.
